Reworking vmmeter

[Konstantin Belousov]

On Sun, Jul 07, 2013 at 09:53:37AM +1000, Benno Rice wrote:
> So I've put together this patch:
> 
> http://people.freebsd.org/~benno/vmmeter.diff
> 
> This patch does a few things:
> 
> - Renames the singleton "cnt" to "vmmeter".
> - Replaces all the per-cpu counters with counter_u64_t.
> - Removes the vmmeter instance from struct pcpu, due to the above mentioned change.
> - Adds includes for vmmeter.h to a few files that were only getting it via pollution in pcpu.h
> - Removes some entries from assym that weren't being used.
> 
> This has been tested on amd64 and nothing else right now, I'm more posting this to get general comments on whether people think this is a good idea. My motivation for this was twofold, firstly to rename cnt and secondly to move the counters to the common counter framework. More testing will be done prior to commit.
> 

Why did you removed the CTASSERT from sys/pcpu.h ?

Your edits for the inlines in the sys/vmmeter.h are notoriously
violating style.

You probably could add some macro like COUNTER_U64_INITIALIZED(), which
would check for the counter containing non-NULL pointer.  At least this
would allow to remove vmmeter_use_early_counters.  Still the hack of
early_*_faults cannot be avoided this way.  Or, since BSP is guaranteed
to have id 0, you could temporary put a pointer to the early_*_faults
into the counter_u64_t, which is to be overwritten after the real counter
is initialized.  Then the if()s in the vm_fault() can be removed.

Note that the parts of counters(9) KPI used in your patch has known
issue on some 32 bit arches, namely powerpc32, mips32 and arm. The fetch
could loose the carry bit in a cell, transiently. This is a bug in the
platform implementation, and not the inherent counters(9) limitation.
Fixing requires some asm magic (basically, the counter cells updates and
fetches must be 64bit atomics).  This is done on i386 already, and
the problem does not exist on 64bit arches.

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